1. Field of the Invention
This invention relates to a code plate and, more particularly, to a code plate for a camera comprising parts arranged in various two-dimensional shapes which are highly suited for an automatic focus control system for a zoom lens.
2. Description of the Prior Art
In the conventional automatic focus control system of a camera, a code plate is disposed on the lens side with the code plate consisting of signal source elements of different kinds, such as conductive parts and non-conductive parts. These parts are alternately arranged in a given direction to form a continuous comb-like on-and-off pattern. The shifting extent of the lens is monitored by allowing a brush to slide over this on-and-off pattern in association with the shift of the lens. The on-and-off pattern elements are spaced to correspond to a prescribed shifting pitch of the focal plane of the lens. For example, when a distance measuring unit of the camera detects that the focal plane deviates 1 mm from the surface of film, a lens position shifting motor is driven to shift the lens to a position which is reached with a total of a predefined number (e.g. 20) of on-and-off signals produced from the code plate and the lens is thus set in an in-focus state at that position.
In the case of photographing with a zoom lens, the ratio of the lens position shifting extent to a focal plane shifting extent varies with the zooming ratio. When the zooming ratio is for a double magnification, for example, the focal plane shifting extent resulting from the same front lens shifting extent varies to provide a four-fold difference between the wide angle side and the tele-photo side of the lens. More specifically, if one on-and-off signal corresponds to a focal plane shifting extent of 50.mu. on the wide angle side of the lens, it comes to correspond to a focal plane shifting extent of 200.mu. on the tele-photo side of the same lens. Accordingly, with the above-stated conventional code plate employed, the spacing distance of the on-and-off pattern greatly deviates from the prescribed pitch of the focal plane shifting extent when the zooming ratio of the lens becomes large. In that event, a focusing action would be repeated over and over again or result in hunting.
Conceivable solutions of this problem include an arrangement to electrically frequency divide the on-and-off signal according to the zooming ratio. However, this arrangement involves difficulties in terms of signal processing and electrical connection between the lens and the camera body.